The ever-increasing miniaturization of electronic components modules and assemblies and the market pressures for cost reduction has made the assembly of those devices a precise, automated, multi-step task. Most devices are assembled using surface mount technology (“SMT”) wherein scores, if not hundreds of individual components are precisely placed and soldered on at least one printed circuit board in an “assembly line” fashion.
Printed circuit boards travel successively, in-line along conveyors through a series of stations which perform each step in the assembly process. Typically, an empty board enters a solder paste delivery system which places uncured solder paste on portions of the board requiring soldered connections. The board then enters one or more chip shooter stations which physically place components on the board. The board then proceeds through an oven which cures the solder paste. After cooling the board is ready for testing and other finalization steps prior to packaging and shipment.
At each step there is a potential for errors to occur which result in a defective board. Some of the potential printed circuit board assembly defects include: circuit board defects such as opens and shorts on the traces; placement defects wherein components are missing, of the wrong type, incorrectly oriented, or misaligned; solder defects in amount and placement which can result in solder bridges on the leads or tomb-stoning of components caused by solder contraction during curing; and other defects such as damage caused by mechanical mishandling.
Previous procedures and devices for testing whether defects exist on freshly assembled printed circuit boards suffer from various drawbacks.
Human testing and inspection is costly, slow and subject to a high degree of inaccuracy. The devices used by human testers are typically heavy, bulky, and not readily portable. Electronic in-circuit testing suffers from being slow and highly iterative in order to pinpoint the location of a defect and often cannot detect the most common manufacturing errors.
In order to minimize continued work on a board which has already become defective, manufacturers often provide for testing at several stages during assembly. However, a particular piece of automated test apparatus is usually designed to test a specific type of board, specific defects, and/or only at a specific point in the assembly. Therefore, numerous different testing devices have been required.
Current automatic visual or other electromagnetic radiation based inspection systems suffer from similar drawbacks. X-ray based systems are suited to scan for metallic defects such as faulty traces and subsurface defects. However, high resolution x-ray inspection is expensive and time consuming, and potentially hazardous to nearby human operators.
In other systems, light produced by lamps or LEDs (“Light-Emitting Diodes”) is reflected off the surface being inspected into one or more video cameras. Some require the use of two images obtained under different lighting conditions as disclosed in Takahashi, U.S. Pat. No. 5,059,559. Other various digital and analog signal analyzing processes can be used to determine the existence of visually detectable defects. For example analyses have been made upon a monochrome intensity comparison measurement of the signal corresponding to the image of the gaps between terminal leads.
These systems are relatively low resolution and hence slow. If thorough inspection is required, the system must zoom in and successively scan portions of the board in a piecemeal fashion. In addition, monochrome intensity comparisons are prone to inaccuracies where adjacent features have similar intensities. An averagely populated, 3 inch by 5 inch board, such as a standard PCI SVGA video adapter card will take about 1 minute 20 seconds to inspect thoroughly.
Most prior systems require extremely precise location of the board and camera, on the order of 0.001 of an inch. The board and camera must be made resistant to vibration. The prior solution entailed a massive platform made of slate or other heavy materials, and precise, vibration-resistant board handling and camera carriage mechanisms. Most prior systems weighed greater than 450 kilograms. These requirements increase the cost and lower the portability of the system.
Therefore, it is desirable to have an economical, automated testing system, which quickly detects the existence of the most prevalent manufacturing defects, which determines automatically whether a particular board may benefit from reworking and efficiently informs the rework station of those defects; which keeps track of defects over time to identify problems symptomatic to the assembly system; and which is quickly and easily moved to different points in the assembly line or out of the assembly line altogether for manual testing.
The instant invention results from an attempt to reduce cost, and to improve the throughput and efficiency of automated assembly systems.